1. Field of the Invention
The present invention relates to processes making it possible to program integrated memories, particularly those provided for smart cards, for recording therein binary data of various types.
2. Description of the Prior Art
Smart cards, also known as chip cards, are cards of the same size as a credit card and in which is inserted at least one integrated circuit having a memory and logic processing circuits which can optionally constitute a microprocessor. At least part of the memory is programmed in order to form a read-only memory (ROM), so as to e.g. be able to personalize the card. For this purpose the memory is programmed and then a fuse is blown, which makes it possible to prevent any writing in the thus programmed area. Thus, said area can be read, but cannot be reprogrammed. In principle, the blowing action of a fuse leads to a very reliable protection, because normally a fuse is not reconstituted. However, in practice, although it is not reconstituted, the blown fuse returns to a relatively conductive state, which can then permit the reprogramming of the normally protected area. The reason for this is not known and it is believed that particles of conductive elements reconstitute over a longer or shorter term the destroyed connection, as a result of the minute dimensions of said connection. As a result of the very small dimensions, for the purpose of blowing the fuses, the breakdown voltage is only applied for a short time of between 5 and 10 ms and the maximum intensity of the breakdown current is limited to approximately 30 mA, so as not to risk damaging the connections adjacent to that which has been blown.
The devices known at present for blowing fuses are very simple and are essentially dedicated to one circuit type. Therefore they do not make it possible to modify the breakdown voltage to be applied to the circuit or the time during which said voltage is applied. Moreover, in order to respect the limits given above, the current supplied is low. Finally, the breakdown is not systematically verified.